1. Field of the Invention
The present invention relates to visible radiation-curable solder resist compositions and a method for forming solder resist patterns. More precisely, it relates to visible radiation-curable solder resist compositions for forming protective films, which are to form protective films over the regions to be protected or insulated in, for example, printed circuit boards with conductor circuits formed thereon, for the purpose of protecting the conductive metals in said printed circuit boards from being oxidized and for the purpose of electrically insulating said conductive metals therein, and relates to a method for forming solder resist patterns for said purposes.
2. Related Art
Two conventional patterning methods are known, one being a screen-printing method and the other being a photo-printing method using ultraviolet rays.
The screen-printing method is advantageous in that its producibility is high and that it is inexpensive, but is disadvantageous in that its printing accuracy and reproducibility are poor thereby limiting the printable latitude to a line width of 0.2 mm or thicker.
The photo-printing method using ultraviolet rays is advantageous in that its printing accuracy and reproducibility are good, but is disadvantageous in that its producibility is low. In addition, as using ultraviolet rays, this method is problematic in the following points.
Photosensitive compositions are used in the conventional photo-printing method, while serving plenty of uses for, for example, photoresists, photolithographic and photoengraving materials, PS plate materials for offset printing, information-recording insulating materials, and solder resists. Many of such photosensitive compositions are sensitive to ultraviolet rays. However, since their sensitivity is very small to fall generally between several tens and several hundreds mJ/cm.sup.2, they require high-power light sources. In addition, in the photo-printing method, masks are used for the exposure to ultraviolet rays, since the energy conversion in the UV exposure is poor.
On the other hand, another imaging method is known, which uses laser rays having a high energy density for directly forming images. This method is advantageous in that the energy conversion therein is good and that the imaging step constituting it can be significantly simplified.
It is desirable to use visible lasers having a longer life and having more stable light intensity than ultraviolet lasers, as scanning exposure sources for direct imaging. Therefore, desired are visible radiation-curable compositions that are sensitive to scanning exposure to visible lasers.
To meet this demand, for example, a photolithographic material sensitive to visible rays has been reported in Japanese Patent Application Laid-Open No. 62-31848; a high-sensitivity photopolymerizable composition comprising a photopolymerization initiator, which is sensitive even to low-intensity ultraviolet rays and visible rays and which can be exposed even to lasers, has been reported in Japanese Patent Application Laid-Open No. 61-233736; and a polymerizable composition which is sensitive to not only ultraviolet rays but also visible rays, especially to light near to 500 nm has been reported in Japanese Patent Application Laid-Open No. 60-221403.
Given the situation as above, there is now another demand for visible radiation-curable solder resist compositions which are sensitive to scanning exposure to visible lasers and which are applicable to the formation of thick films of solder resists. For example, Japanese Patent Application Laid-Open No. 3-179064 discloses a visible radiation-curable resin composition. The composition disclosed therein is used for forming relatively thin etching patterns, and this is removed after etching. However, there is not known any practicable, visible radiation-curable resin composition which can form relatively thick solder resist patterns.
Patterning of solder resists is generally conducted according to a screen-printing method, or to a photo-printing method where an ultraviolet-curable solder resist composition (see, for example, Japanese Patent Application Laid-Open No. 1-141904, and Japanese Patent Publication No. 1-54390) is exposed to ultraviolet rays. However, as mentioned hereinabove, these methods are problematic in various points.